On Firewalla, lo interface is excluded in all dnsmasq settings of all
interfaces, to prevent conflicts. The one that ctrld adds in
dnsmasq_local directory could not work if there're multiple dnsmasq
configs for multiple interfaces (real example from an user who uses
VLAN in router setup).
Instead, if we detect 127.0.0.1 on Firewalla, fallback to "br0"
interface IP address instead.